Transient Voltage Suppressor Diodes (TVS) Information
Transient voltage suppressor (TVS) diodes are designed to limit over-voltages. They can dissipate high amounts of transient power in a short period of time. The sharp response of the diode's breakdown voltage acts as a means to quickly dissipate transient voltages to a ground.
Transient Voltage Waveforms
Transient voltages are undesired surges or spikes of electrical energy. While the duration of these events is small, approximately one millisecond, the amount of power released in these sudden events can cause permanent damage to electrical circuits. Lightning, electrostatic discharge (ESD), switching of inductive loads, or other electro-magnetic interferences (EMI) can all cause transient voltages. Waveforms can be classified by the source or timing of the transient. Common transient waveforms include: 10/1,000μsec, 8/20μsec, inductive load transients, and lightning induced transients.
The 10/1,000μsec waveform is an industrial-standard test condition used to test the TVS diodes power rating. The surge absorbing capability of the diode is tested by applying the peak impulse current (IPPM) at 10μsec and dropping to half of the peak impulse current (IPPM) at 1,000μsec.
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The 8/20μsec waveform is an industrial-standard test condition used to test the TVS diodes capability to absorb electrostatic discharges (ESD). The capability of the diode is tested by applying the peak impulse current (IPPM) at 8μsec and dropping to half of the peak impulse current (IPPM) at 20μsec.
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Inductive Load Transients
Inductive loads are capable of generating high energy transients. When an inductive load is removed, the collapsing magnetic field is converted into electrical energy. The resultant waveform is a double exponential transient. Depending on the application there can be a range of wave shapes, durations, peak currents, and peak voltages. These values need to be approximated in order to select a suitable TVS diode.
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Lightning Induced Transient
Lightning induced transient waveforms produce a single peak above the line voltage which accounts for the majority of the energy dissipation from an impulse. The duration of lightning induced transients is very short and the peak impulse current (IPPM) may be observed on the magnitude of hundreds of nanoseconds to several microseconds.
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When a transient voltage greater than the breakdown voltage of the TVS is applied, the device routs the current generated to ground. This is a similar operation as performed by a clamper circuit. In fact, a TVS diode is considered a clamper. When the voltage returns to levels below the breakdown voltage, the TVS diode ceases to conduct current to ground and the circuit returns to normal operation.
Some TVS diodes work in both current directions. Accordingly, we classify the TVS diodes as unidirectional or bidirectional:
• Unidirectional. The TVS diode provides protection when current moves in only one direction.
• Bidirectional. The TVS diode provides protection when current moves in both directions.
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TVS diodes should be sized so that they can sustain foreseen transient pulses without failure and clamp the transient voltage without interrupting the line voltage. Common specifications used to select a transient voltage suppressor diode include the peak pulse power, maximum working voltage, maximum peak impulse current, clamping voltage, peak forward surge current, breakdown voltage, terminal capacitance, and operating temperature.
Peak Pulse Power (PPP) is the maximum power that transient voltage suppressor diodes can suppress without failure. The size of the diode will be proportional to the peak pulse power that it is designed to handle.
Maximum Working Voltage (VWM) is the continuous (DC) or peak voltage that can be applied to the device over the operating temperature range. When selecting a TVS, this voltage should be larger than the voltage of the device to be protected. The maximum working voltage, also known as standoff voltage, will not be directed through the diode to the ground.
Clamping Voltage (VC) is the observed voltage across the suppressor when it is conducting the IPPM peak impulse current.
Peak Impulse Current (IPPM) is the rated maximum value of the peak impulse current (IPP) that the suppressor can withstand.
Breakdown Voltage (VBR) is the reverse voltage at which a small increase in voltage results in a sharp rise of reverse current.
Terminal Capacitance (CT) is the terminal capacitance when the specified reverse voltage and frequency are applied. It is also known as diode capacitance (CD).
Junction Operating Temperature (Tj) is the range of temperatures over which the diode is designed to operate.
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Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. RoHS became effective on July 1, 2006.